High-strength steel sheets are used in wide-ranging uses such as automobiles, transports, household electrical appliances, and building materials. Automobiles and transports, for example, desirably have smaller weights for lower fuel consumption. Among them, automobiles require collision safety, and structural parts such as pillars, and reinforcing parts such as bumpers and impact beams for use in the automobiles should have higher strengths. Of these, members requiring rust prevention also employ hot-dip galvanized steel sheets (hereinafter also referred to as GI steel sheets) and hot-dip galvannealed steel sheets (hereinafter also referred to as GA steel sheets) because of excellent rust prevention of the GI steel sheets and GA steel sheets. The GA steel sheets are manufactured by subjecting GI steel sheets to an alloying treatment. A steel sheet, when designed to have a higher strength, may have inferior elongation (ductility) and thereby have poor workability. To prevent the deterioration in workability, the aforementioned steel sheets require good balance between strength and elongation and also require good bending workability without cracking upon working.
Patent literature (PTL) 1, PTL 2, PTL 3, and PTL 4 disclose techniques for improving the workability (strength-elongation balance and bending workability) of high-strength steel sheets. Of these, PTL 1 discloses a high-strength GI steel sheet having a tensile strength of 780 MPa or more and having improved bore expandability and bendability, in which the steel sheet has a metal structure including 50% or more of a ferrite phase and 10% or more of a martensite phase, the ferrite phase includes a bainitic ferrite phase in an amount of from 20 to 80 percent by area, and the martensite phase has an average grain size of 10 μm or less. Specifically, the steel sheet contains a highly ductile and soft ferrite phase in an amount of 50 percent by area or more to ensure satisfactory ductility, and contains chromium (Cr) in a large amount to increase the amount of martensite as a second phase to thereby ensure a satisfactory strength.
PTL 2 discloses cold-rolled thin steel sheet which includes 50 to 90 percent by volume of a martensite phase, 5 to 35 percent by volume of a hard bainite phase, 35 percent by volume or less of a soft bainite phase, and 0.1 to 5 percent by volume of retained austenite, has a tensile strength of 1100 MPa or more, and has a bore expansion ratio of 40% or more. The cold-rolled thin steel sheet, however, probably fails to have both pod strength-elongation balance and satisfactory bendability, because the steel sheet has a low elongation because of the presence of the hard bainite phase. In addition, the cold-rolled thin steel sheet requires production facilities to perform slow cooling and rapid cooling in combination so as to obtain the hard bainite phase, resulting in high cost.
PTL 3 discloses a high-strength steel sheet exhibiting excellent formability and having a tensile strength of 980 MPa or more. This high-strength steel sheet is designed to have a higher strength by utilizing a martensite structure, to contain carbon (C) in a content of 0.16% or more, and to utilize transformation of upper bainite. The steel sheet therefore includes a sufficient amount (specifically 5% or more and 50% or less) of retained austenite which is stable and is advantageous for obtaining transformation induced plasticity (TRIP) effects.
PTL 4 discloses a high-strength steel sheet having a tensile strength of 800 MPa or more and exhibiting satisfactory bore expandability, as a steel sheet containing niobium (Nb) and molybdenum (Mo) in combination and having a specific metal structure. The metal structure contains a total of 70% or more of one or more phases selected from bainite, bainitic ferrite, and a martensite having a carbon content of less than 0.1% or having a Vickers hardness of 450 or less and contains, if any, retained austenite in a controlled amount of less than 3%.